I simply think of these as being fundamentally different things. An asteroid is a rock, or a collection of rocks, with no particular form and no history of geological activity. Spherical rocky objects have a history of geological process driven by their own gravity that causes the colliding rocks that formed them to change form and through these processes end up with the classical core, mantle and crust formation. I think this makes them fundamentally different to rocks that just happened to stray into the gravitational path of a planet and become trapped and forever unchanged other than a few more craters over time.

So the DRT classification is as follows.
• Things big enough to be rounded:
◊ If in orbit around the sun are ‘planets’ (about a dozen are known, there might be hundreds);
◊ If in orbit around something orbiting the sun are ‘moons’ (there being about 19, the smallest perhaps being Mimas).
• Things too small to be rounded are ‘rocks’, ‘pebbles’ or ‘dust’.

jdaw1 wrote:So the DRT classification is as follows.
• Things big enough to be rounded:
◊ If in orbit around the sun are ‘planets’ (about a dozen are known, there might be hundreds);
◊ If in orbit around something orbiting the sun are ‘moons’ (there being about 19, the smallest perhaps being Mimas).
• Things too small to be rounded are ‘rocks’, ‘pebbles’ or ‘dust’.

I may regret asking this, but what is it about the 'rules' above that some disagree with, and thus conclude that Pluto is not a planet? That sort of means I am assuming Pluto meets the definition above of being rounded and orbiting the sun...is that true?

jdaw1 wrote:So the DRT classification is as follows.
• Things big enough to be rounded:
◊ If in orbit around the sun are ‘planets’ (about a dozen are known, there might be hundreds);
◊ If in orbit around something orbiting the sun are ‘moons’ (there being about 19, the smallest perhaps being Mimas).
• Things too small to be rounded are ‘rocks’, ‘pebbles’ or ‘dust’.

I may regret asking this, but what is it about the 'rules' above that some disagree with, and thus conclude that Pluto is not a planet? That sort of means I am assuming Pluto meets the definition above of being rounded and orbiting the sun...is that true?

The classification above relates to a discussion on what constitutes a moon, not what constitutes a planet.

The debate about planets is set out on the previous three pages of this thread but I think can be fairly summarised as follows:

1. Some people, DRT included, think that being a sphere and orbiting the Sun is enough to be called a planet.

2. Others, jdaw1 included, think that some spheres are too numerous, far away and small and should therefore not be called planets.

Pluto falls into category 1 as a planet and category 2 as not a planet.

To be a moon, a body must be:
• In direct orbit around a body too small to be a star;
• Have been sighted multiple times within a mapped orbit;
• Be larger than a chunk of Westcombe Cheddar as served by the Boot & Flogger.

jdaw1 wrote:To be a moon, a body must be:
• In direct orbit around a body too small to be a star;
• Have been sighted multiple times within a mapped orbit;
• Be larger than a chunk of Westcombe Cheddar as served by the Boot & Flogger.

I agree with your first point.

I disagree with your second point on the basis that a moon is still a moon even if we or another species have not yet seen it.

I disagree with your third point on the basis that there is too wide a gap between a chunk of cheese and something large enough to be considered a moon. Your description implies that every piece of space junk currently orbiting the earth is a moon, along with all of the rocks too small to be detected.

jdaw1 wrote:To be a moon, a body must be:
• In direct orbit around a body too small to be a star;
• Have been sighted multiple times within a mapped orbit;
• Be larger than a chunk of Westcombe Cheddar as served by the Boot & Flogger.

I agree with your first point.

I disagree with your second point on the basis that a moon is still a moon even if we or another species have not yet seen it.

I disagree with your third point on the basis that there is too wide a gap between a chunk of cheese and something large enough to be considered a moon. Your description implies that every piece of space junk currently orbiting the earth is a moon, along with all of the rocks too small to be detected.

I agree with DRT on all points.

But that does pose a further question: what to call the (now former) moons of Mars? Phobos and Deimos are not round, so do not qualify as moons. I've always thought it weird that they were considered moons. They are thought to be captured asteroids. Do we now refer to them as captured asteroids? Or do we simply expand what "asteroid" means to include non-moon planet-orbiting bodies? Or are they dwarf moons? Or do we simply grandfather them in as moons?

To me there is a clear category of planet-orbiting objects larger than junk/rocks/the components of Saturn's rings yet smaller than a true moon.

I do not claim that my definition is or should be universally accepted, I just think that planet-like moons are so materially different to the captured asteroid or big rock type moons that they deserve a special category.

I have not studied the moons of Mars but do realise they are not spheres. Could it be that one day they will be? Are they of sufficient size that their own gravity and the forces exerted upon them by their host planet will pull them into shape?

DRT wrote:I have not studied the moons of Mars but do realise they are not spheres. Could it be that one day they will be? Are they of sufficient size that their own gravity and the forces exerted upon them by their host planet will pull them into shape?

You do realize that you are asking for jdaw1 to quote math at us again, right?

DRT wrote:I have not studied the moons of Mars but do realise they are not spheres. Could it be that one day they will be? Are they of sufficient size that their own gravity and the forces exerted upon them by their host planet will pull them into shape?

You do realize that you are asking for jdaw1 to quote math at us again, right?

I've been trying to tempt him for a whole page of posts and you've just scared him off again!